Force and Motion Online Test 7th Science Lesson 2 Questions in English

The shortest distance from the initial to the final position of an object is called Displacement.

When an object travels from one place to another, it will reach faster if it travels along the straight-line path. The straight-line path is the shortest distance between two points.

Both the distance and displacement possess the same unit. The SI unit is meter (m).

The path of an object travelling from A to B is shown in figure. Total distance travelled by the object is 120 m. The displacement of the object is 40 m (south-east direction). Here we can consider the starting point as A and while the object moves from A to B the displacement is considered to be positive and from B to A it is negative.

1 km/h = 5/18 m/s
1 km = 1000 m; 1 h = 3600 s
1 km / h = 1000 m / 3600 s = 5/ 18 m /s.

Speed is the rate of change of distance. Speed = distance /time Unit is metre/second (m/s).

If a body covers unequal distances in equal intervals of time, the body is said to be in nonuniform speed. Average speed = total distance travelled / time taken to travel the distance.

Velocity is the rate of change in displacement. Velocity (v) = displacement / time.

Speed = distance / time = 200 / 25 = 8 m/s
velocity = displacement /time = 50 / 25 = 2 m/s.

SI unit of velocity is meter / second (m/s).

A body has uniform velocity, if it covers equal displacement in the same direction in equal intervals of time. E.g., light travels through vacuum.

If either speed or direction changes, the velocity is non uniform. E.g., a train starting and moving out of the station.

Average velocity = total displacement / total time taken.

Average velocity = total displacement/time taken. (taking the direction due east of point O as positive)
= (5 – 7) / 0.2 = –2 / 0.2
= –10 km/h (or) –10 × 5/18 = 25/9
= – 0.28 m/s

The triangle method can help you to recall the relationship between velocity (v), displacement (d), and time(t). v = d / t, t = d / v, d = v × t.

Acceleration is the rate of change in velocity. In other words, if a body changes its speed or dirction then it is said to be accelerated.

Acceleration = change in velocity/ time
= [final velocity (v) – initial velocity (u)] / time (t)
a = (v–u) / t.
In other words, the object undergoes acceleration when its speed and/or direction change(s).

SI unit of acceleration is m/s2.

Initial velocity u = 0 m/s (since the car starts from rest)
Final velocity (v) = 12 m /s
Time taken (t) = 4 s
acceleration (a) = (v – u) / t
= (12 – 0) / 4
= 3 m / s2

If the velocity of an object increases with respect to time, then the object is said to be in positive acceleration or just acceleration.

If the velocity of an object decreases with respect to time, then the object is said to be in negative acceleration or deceleration or retardation.

Initial velocity (u) = 8 m/s
Final velocity (v) = 2 m/s
Time taken(t) = 10 s
Acceleration (a) = (v – u)/t = (2 – 8)/10 = –0.6 m/s2
The deceleration is –0.6 m/s.

The centre of gravity of an object is the point through which the entire weight of the object appears to act. Try to balance a cardboard on your figure tip. We observe there is only one point which the cardboard is balanced. The point which the cardboard is balanced is called the centre of gravity of the cardboard.

Stability is a measure of the body’s ability to maintain its original position.

The three types of stability are (a) Stable equilibrium (b) Unstable equilibrium (c) Neutral equilibrium.

In this unstable equilibrium the frustum will topple with the slightest tilting. Its centre of gravity is lowered when it is displaced.

The vertical line through its centre of gravity falls outside its base. It causes frustum to topple. The frustum will roll about but does not topple. Its centre of gravity remains at the same height when it is displaced. Te body will stay in any position to which it has been displaced.

The Thanjavur Doll is type of traditional Indian toy made of terracotta material. Th e centre of gravity and the total weight of the doll is concentrated at its bottom most point, generating a dance-like continuous movement with slow oscillations.

To make a body more stable 1. Lower its centre of gravity 2. Increase the area of its base 3. Th is box is at the point of tipping over 4. A heavy base lower at the centre of gravity So the box does not tip over 5. A broad base makes the box more difficult to tip over.

The frustum can be tilted through quite a big angle without toppling. Its centre of gravity is raised when it is displaced. The vertical line through its centre of gravity still falls within its base. So it can return to its original position.